here are some fairly good reasons in favor of Russia's decision to intervene in Syria, which is why I have always been modestly if unenthusiastically supportive of it:
It is basically a giant and continuous live training exercise for Russian pilots and generals, making it almost "free" in financial terms.
The value of the Khmeimim base is modest, but not entirely negligible.
It supported Russian weapons sales.
Fighting Islamic State made for good PR.
Could potentially be used as a bargaining chip for concessions elsewhere (e.g. the Ukraine).
One commonly cited but fake reason: Supporting an ally. As I have long been pointing out, it was Vladimir Putin himself who pointed out that prior to the war, Assad had visited Paris more frequently than Moscow.
However, there were always a couple of major downsides:
Supporting Assad placed Russia at odds with all of the powerful players in the region – the US, its European allies, Israel, Saudi Arabia and the Gulf Arabs, and Turkey. The only exception was Iran, and even its interests are far from synonymous with Russia's.